Field of the Invention
The present invention relates to an illumination optical system that includes an illumination optical element that illuminates an illumination target surface in a planar manner, and an illumination apparatus, and further relates to the illumination optical element that illuminates an illumination target surface in a planar manner.
Description of the Related Art
In the fields of microscopy and semiconductor exposure apparatuses, Kohler illumination is generally applied for illuminating a specimen surface uniformly. However, Kohler illumination causes non-uniformity of illumination due to characteristics in which an intensity of light emitted from a light source varies by exit angle (hereinafter referred to as “light distribution characteristics”). Thus, a technology to suppress non-uniformity of illumination caused by the light distribution characteristics of the Kohler illumination by use of fly eye lenses has been proposed. Such a technology is described, for example, in Japanese Laid-open Patent Publication No. 2004-4169.